The IEC reactor is a vacuum chamber filled with a fuel gas such as deuterium at low pressure. There are inner and outer spherical or cylindrical grids centered inside the chamber. The outer grid is held at nearly zero potential, and the inner, 90-99% transparent grid is held at a high negative potential, typically -100 kV. The potential difference between the grids accelerates ions inward to velocities of fusion relevance.
Cross sections versus center-of-mass energy for key fusion reactions.